1. Field of the Invention
This invention relates generally to tubular shafts. More particularly, the present invention involves tapering a co-extruded tubular shaft (e.g., for use in catheters).
2. Description of Related Art
In percutaneous transluminal coronary angioplasty (PTCA), catheters are inserted into the cardiovascular system via the femoral artery under local anesthesia. A pre-shaped guiding catheter is positioned in the coronary artery, and a dilatation catheter having a distensible balloon portion is advanced through the guiding catheter into the branches of the coronary artery until the balloon portion traverses or crosses a stenotic lesion. The balloon portion is then inflated with a fluid to compress the atherosclerosis in a direction generally perpendicular to the wall of the artery, thereby dilating the lumen of the artery. A standard catheter having a relatively constant diameter is difficult to use in some of the smaller arteries and in cases of more advanced stenosis where the artery is closed to such an extent that the catheter cannot be extended through the lesion. Thus, a catheter having a narrowing or a tapered shaft will be beneficial in many circumstances.
A conventional dilatation catheter 5 known in the art for use in treating angioplasty is illustrated in FIG. 1. Typically, dilatation catheters are co-axial catheters having a cross-section such as the one illustrated in FIG. 2. FIG. 3 is a side cross-sectional view of catheter 5 of FIGS. 1 and 2. Catheter 5 has a catheter shaft having an inner member 10 extending through an outer member 12. Typically, outer tubular member 12 is sealed to the proximal shaft of a balloon 14, while inner tubular member 10 is sealed to the distal shaft of balloon 14. Fluid for inflating balloon 14 coupled to the distal end of inner 10 and outer 12 members is introduced through a passageway 11 formed between the tubular members (i.e., the outer lumen). A guide wire (not shown) then passes through a central opening or lumen 13 of inner tubular member 10.
Due to the different responsibilities of each tubular member, inner 10 and outer 12 tubular members generally have different desired material characteristics, which complicates the selection of materials for each of the various catheter components. Typically, outer member 12 is fusion bondable to another catheter component and inner member 10 has a greater lubricity than outer member 12 to allow for ease of passage by the guidewire through inner catheter shaft 13. For example, the material of outer member 12 of the catheter shaft must be selected such that it is compatible with the polymeric material (e.g., polyethylene, terephithelate, polyamide, nylon, etc.) of the catheter component to which it is to be secured (e.g., a balloon or transition piece). Furthermore, because outer member 12 of the catheter shaft is in contact with a patient, the material must be non-traumatic to the lining of the arterial walls into which the catheter is inserted. In contrast, inner member 10 is generally selected for its lubricious properties to allow easier passage of the guide wire through inner lumen 13. Note that the same issues as identified above exist for a catheter shaft having a single tubular member: e.g., the interior of the member should be lubricious and the exterior should be both non-traumatic to the patient and bondable to another catheter component. Thus, when dealing with a single tubular member the material selection is particularly difficult.
One solution for addressing the difficulties in material selection of the more recently developed catheters has been the production of an improved multi-layer member fabricated by a co-extrusion process. A multi-layer member may be used independently as a sheath or as a catheter having a single tubular member (i.e., a single lumen catheter), or in conjunction with a second tubular member to form a co-axial catheter. When used with a co-axial catheter, either or both of the inner and outer tubular members may be a multi-layer member.
A multi-layer member may have many layers or as few as two layers, but typically consists of two to three layers. A cross-sectional illustration of a three layer tubular member 28 is shown in FIG. 4. An inner layer 20 is typically lubricious such that a guidewire or other device may easily be inserted through an interior lumen 26. An outer layer 24 is fabricated from a material that may easily be bonded to another component, e.g., a balloon, and that is strong enough to resist collapse pressure. A middle layer 22 is generally an adhesive or compatibilized polymer used to enhance the integrity of member 28.
The formation of multi-layer tubular member 28 may be achieved through a co-extrusion process using multiple extruders. Generally, the materials selected for each layer are first processed in separate extruders. Each of the selected materials is separately brought to a molten state. Then the materials are brought together to form a single hollow tubing having inner layer 20 from a first material, middle layer 22 from a second material, and outer layer 24 from a third material.
Another complicating factor in the selection of materials for the various components of catheters is the usual requirement that the proximal shaft section (16 of FIG. 1) be much longer and more rigid or stiff than the distal shaft section (18 of FIG. 1) such that proximal shaft section 16 provides pushability to catheter 5. This allows the more flexible distal shaft section 18 to be readily advanced through an often tortuous anatomy. Thus, a stiff proximal shaft may often be joined to a soft distal shaft. Note that the proximal shaft may be made stiffer than the distal shaft due to a larger (i.e., thicker) diameter or through use of a stiffer proximal polymeric tubing made from polymers such as PEEK. The joint between the proximal and distal shafts, however, often makes for an undesirable, abrupt transition between shaft sections.
There are numerous methods of establishing a connection or joint between the stiff proximal shaft and the more flexible distal shaft such as laser powered fusion of polymeric materials. The abrupt transition caused by joining a larger diameter proximal shaft to a smaller diameter distal shaft not only creates a weak point on the catheter shaft at the point of joinder, but the joint region itself is typically stiff and can interfere with the overall flexibility of the catheter shaft. (See transition 30 between a proximal shaft 32 and a distal shaft 34 in FIG. 5). Also, the fluid flow is typically retarded at a transition point 30, which increases the deflation time. Thus, a more gradual tapering of a single catheter shaft would provide several benefits over a catheter shaft having an abrupt transition between the proximal and distal shaft sections.
One solution to providing a tapered catheter shaft has been to “neck down” a small section of a straight shaft, as illustrated in FIG. 6. The straight shaft is submitted to post-process heating (i.e., after the shaft is formed) and then pulled in a controlled manner (i.e., necked down or stretched out). This necking process provides a necked shaft having a tapered appearance, which allows for a more flexible distal section. However, there are several limitations and problems associated with the necking down process. First, the necking down process is usually able to taper only a short section 40 of the shaft. For example, the standard length of the necked down region is approximately 2–10 cm. Thus, if a more gradual taper is desired, for example a taper of the shaft over a longer region (e.g., 1 ft.), a method other than or in addition to necking must be used to taper the shaft. Second, a relatively immediate change (i.e., 3 cm) such as transition 30 illustrated in FIG. 5 can still result at the point of necking (i.e., where the heat is applied), resulting in a weak spot in the catheter. Such weak spots are more susceptible to leaks, kinks and cracks, which adversely affect the reliability of the catheter. Further, the necking process, in addition to reducing the diameter of a short section of the shaft, also results in a stiffer and less desirable shaft.
With multi-layer tubular shafts, the necking process used in the prior art to decrease the diameter of the catheter shaft has several additional and significant disadvantages. For example, outer layer materials are not typically fully compatible with inner layer materials and respond differently to the heat applied during the necking process. Thus, the risk of separation between adjacent layers is increased. In other words, it is often difficult for adjacent layers of a co-extruded multi-layer tubular member to remain bonded during necking as the high strain rate promotes separation of the layers. This is problematic because one layer may “recover” back (i.e., expand when heated and then return to the original size or smaller when cooled). This change to the physical and/or chemical characteristics of the layers of a multi-layer tubular member may cause dimensions of the tubular member to change inconsistently among the various layers or induce broken multi-layer bonds. Because the choice of materials for use in the layers of a multi-layer catheter is already limited, further constraints required by the current necking down process become unduly prohibitive.
Another method of tapering a shaft known in the art is extrusion tapering. Extrusion tapering has several benefits over the above-described necking down process. For example, extrusion tapering may produce a more gradual taper over a longer section of the catheter shaft rather than being limited to a short section of approximately 2–10 cm as with the necking down process. The more gradual taper reduces the occurrence of weak points along the catheter shaft resulting from an abrupt change in shaft diameter. Second, separation is not an issue because the tapering of the shaft is done concurrently with the formation of the tubing itself in the molten state of the polymers, rather than post-process in the solid or transition state of the polymers. Note that extrusion tapering in the prior art has been limited to the fabrication of a tapered single-layer tubular shaft.
A method of providing a multi-layer tube having a gradually tapered shaft is desirable. It is also desirable to replace the necking process using heat with another process that will prevent, or substantially reduce, separation between adjacent layers in a multi-layer tube.